Abstract: Clubroot, caused by Plasmodiophora brassicae, is a devastating disease in cruciferous crops and the problem is increasing, among others because the pathogen is very difficult to control. Therefore, new alternative control measures are needed and biological control could be such an option. Clonostachys rosea, isolate IK726, is a versatile fungal biocontrol agent and it was tested as seed coating against P. brassicae in two cultivars of oilseed rape. C. rosea IK726 reduced mean disease severity in both cultivars and furthermore also the frequency of plants with visible disease symptoms. Quantification of pathogen biomass by qPCR of P. brassicae DNA additionally showed that IK726 reduced this parameter and reductions were seen as early as 7 days after pathogen inoculation. Collectively, the results presented here show that C. rosea isolate IK726 can control clubroot under controlled conditions and that it could be considered a potential candidate for future field testing.